Quasi-static and high strain rate fracture behaviour of Ti6Al4V

Abstract

Static and dynamic experiments on sheet and bar titanium Ti6Al4V alloy are presented. Aiming at a comprehensive set of data on the materials damage and failure properties, different sample geometries and loading modes are considered. For the Ti6Al4V sheet, tensile loading of in-plane shear, two different in-plane tensile and plane strain samples are used, for the bar, tensile loading of cylindrical samples and torsion of thin-walled tubes. High strain rates, up to 1500s−1, are achieved in split Hopkinson bar tensile and torsion setups. The evolution of the surface strain fields in the samples is obtained by a digital image correlation technique. Insight is gained in the underlying fracture mechanisms by post mortem evaluation of fracture strains, fracture surface morphologies and voids in the material adjacent to the fracture surfaces, using optical and SEM micrographs. Further analysis and interpretation of the test results is supported by finite element (FE) simulations which give valuable information on the stress and strain state at and in the vicinity of the fracture location, including the loading path up to the onset of fracture. In the FE simulations, strain rate and temperature dependent hardening is taken into account with the Johnson–Cook hardening law. Two models are used to describe the fracture behaviour of the Ti-alloy: for the sheet the phenomenological Johnson–Cook damage initiation criterion combined with an energy-based ductile damage law, for the bar a physically based Gurson-type failure model. Although, neither of the models is able to capture the full complexity of the fracture process, the FE simulations provide key information for an in-depth understanding of the fracture behaviour of Ti6Al4V.In all samples and test conditions, a ductile fracture is observed which is mainly affected by the stress triaxiality. At the lower triaxialities, the process of void nucleation and growth is abruptly ended by strain localization in narrow bands. The effect of strain rate is mainly felt through the induced thermal softening which favours strain localization.